The present invention generally relates to methods of producing bulk forms with controllable microstructures, and more particularly to a large-strain extrusion machining process capable of directly producing bulk forms that have controlled microstructures, including controlled crystallographic textures. The extrusion machining process is capable of forming bulk forms from materials that are typically difficult to fabricate in continuous bulk forms, such as sheets, plates, foils, strips, ribbons, bars, wires, rods, filaments, etc.
Significant benefits can be gained by deforming metals and metal alloys through the application of very large plastic strains. Principal among these are microstructure refinement and enhanced mechanical and physical properties. As an example, there is current interest in the use of “severe” plastic deformation (SPD) to produce bulk forms with controlled microstructures. Such microstructures include, but are not limited to, ultra-fine grained (UFG) microstructures, for example, nanocrystalline structures (NS) characterized by their atoms arranged in crystals with a nominal dimension of less than one micrometer and particularly less than 500 nm.
Multi-stage deformation processing is one of the most widely used experimental SPD techniques for studying microstructural changes produced by very large strain deformation. Notable examples include rolling, drawing, and equal channel angular extrusion (ECAE) processes. In a multi-stage deformation process, very large plastic strains (shear strains of four or more) are imposed in a specimen by the cumulative application of deformation in multiple discrete stages. The effective strain in each stage of deformation is typically on the order of about one or more. However, there are limitations and disadvantages with these SPD processing techniques. A significant limitation is the inability to induce large strains in high-strength materials, such as nickel-based high-temperature alloys and tool steels, as well as materials that are traditionally difficult to deform. Particularly notable examples of the latter include metals having hexagonal close-packed (hcp) structures, including magnesium, titanium and their alloys. Other limitations include the difficulty of imposing strains of much greater than one and the inability to impose strains of much greater than two in a single stage of deformation, the considerable uncertainty of the deformation field, and the minimal control over important variables of the deformation field—such as strain, strain rate, temperature, and phase transformations—that would be expected to have a major influence on the evolution of microstructure and material properties.
U.S. Pat. No. 6,706,324 to Chandrasekar et al. discloses machining techniques for the large scale production of nanostructured (nanocrystalline and UFG) materials. According to Chandrasekar et al., large strain deformation during chip formation in machining is capable of significant grain refinement and development of nanocrystalline and UFG microstructures in a wide variety of materials, including metals and alloys. Chandrasekar et al. teach that, compared to typical severe plastic deformation processes, chip formation during machining can impose very large strain deformation conditions that can be systematically varied over a wide range that is over and beyond what is currently obtainable in other SPD processes. Further advancements based on Chandrasekar et al. include the ability to controllably produce chips with a desired shape and size as disclosed in U.S. Pat. No. 7,628,099 to Mann et al., and the ability to produce continuous bodies with a desired shape and size as disclosed in U.S. Pat. No. 7,617,750 to Moscoso et al. In particular, Moscoso et al. disclose a machining process capable of producing continuous forms having a desired microstructure by simultaneously extruding the chip in the immediate vicinity of the tool cutting edge to produce a monolithic body immediately downstream of the cutting edge. The extrusion process can be controlled to produce monolithic bodies with various cross-sectional shapes and sizes.
Notwithstanding the advancements achieved through the teachings of Chandrasekar et al., Mann et al., and Moscoso et al., further capabilities in the production of bulk forms are desirable. A particular but nonlimiting example is the production of magnesium alloys in bulk forms such as sheets, plates, foils, ribbons, and strips (hereinafter simply referred to as sheet products). Although magnesium is abundant in mineral form and can be refined and cast relatively inexpensively, the creation of sheet products is quite expensive due to the poor workability of this hcp metal. Sheet products are traditionally produced from magnesium ingots by either rolling or extruding into final shape, or by directly casting into sheet forms using twin-roll casting (TRC). Enhanced workability (which, as used herein, refers to the ability to accomplish deformation at low stresses and avoid cracking or other defects) has typically been achieved by preheating magnesium ingots to temperatures above 200° C. to promote basal slip or the activation of non-basal slip. Even if preheated, multiple deformation steps are typically needed to not only reduce the thickness of the sheet, but also to accumulate strain in the material and refine microstructure. In the rolling of magnesium ingots, a 15 to 30% thickness reduction per pass is often imposed in three to seven passes, with intermediate annealing performed between passes. Furthermore, processing speeds are typically low (about 30 m/min) to avoid cracking. If TRC methods are used, roll speeds are minimized in order to achieve uniform solidification and crack-free sheets. The cast sheet is then further processed by conventional rolling or differential speed rolling (DSR) in order to homogenize and refine the cast microstructure and reduce the thickness. Nonetheless, significant material losses are an unavoidable consequence of the high temperature steps and material inhomogeneity. Consequently, production costs for magnesium alloy sheet products are high in comparison to those of conventional alloy sheet products, for example, aluminum and steel alloys.
As known in the art, the lack of workability of magnesium and its alloys is a direct result of an insufficient number of active independent deformation modes required for homogeneous deformation. Main deformation modes active at low temperatures (for example, about 25 to 200° C.) are the basal slip and the {1012} mechanical twinning. An intensive (0002) crystallographic texture develops during conventional rolling that also adversely affects the final workability of magnesium and other low-workability hcp metals. As known in the art, crystallographic texture refers to the degree to which grain crystal axes are aligned within a material. The (0002) crystallographic texture that develops in magnesium is characterized by the basal poles becoming aligned normal (perpendicular) to the rolled surfaces (and therefore the rolling direction) and exhibiting fiber symmetry. The near-zero Schmid factor for the basal slip realized during the forming operations, as a result of this crystallographic texture, results in limited plasticity. On the other hand, workability of magnesium alloys can be promoted by random texture or non-basal (tilted basal) textures, in other words, basal poles tilted at an angle with respect to the normal direction to the sheet product surfaces. However, achieving a random-textured wrought product, especially in hexagonal close-packed metals, is difficult. Grain size is also known to significantly influence the mechanical properties in magnesium and other hcp materials, and finer grain sizes have been shown to enhance both strength and ductility. Though in comparison to conventionally rolling methods, current SPD processes such as ECAE and high-pressure torsion (HPT) are capable of producing finer microstructures and less intensive (0002) crystallographic textures that are essential for subsequent sheet forming or superplastic forming, these techniques cannot be used to make sheet directly from ingot or billet because they do not provide for large shape changes or continuous production.
In view of the above, there is a need for processes capable of producing bulk forms with controllable microstructures, and more particularly processes capable of directly producing continuous bulk forms that have controlled microstructures, including controlled crystallographic textures, from materials that are typically difficult to fabricate in continuous bulk forms.